1. Field of the Invention
The present invention relates to a semiconductor element (typically, a transistor) and a manufacturing method thereof, and more specifically belongs to a technique of a display device using a thin film transistor as a device. That is, the present invention belongs to a technique concerning a display device represented by a liquid crystal display device, an electroluminescence display device, or the like, a technique concerning a sensor represented by a CMOS sensor or the like, and other techniques concerning various semiconductor devices in which a semiconductor integrated circuit is mounted.
2. Description of the Related Art
In recent years, the developments for a liquid crystal display device and an electroluminescence display device in which thin film transistors (TFTs) are integrated on a glass substrate have been progressed. These display devices each are one of semiconductor devices characterized in that thin film transistors are formed on glass substrate using a thin film formation technique and a liquid crystal element or an electroluminescence (hereinafter referred to as just an EL) element is formed on various circuits composed of the thin film transistors, so that a function as a display device is provided.
The circuits composed of the thin film transistors cause unevenness to some extent. Thus, when a liquid crystal element or an EL element is formed on the circuits, a leveling processing using an organic resin film or the like is generally conducted. Each pixel which is provided in a display portion of a display device has a pixel electrode therein. The pixel electrode is connected with the thin film transistor through a contact hole provided in the above-mentioned organic resin film for leveling.
However, the following facts arc found by the studies of the present applicant. That is, when a resin film is used as an interlayer insulating film and a contact hole is formed using a dry etching technique, threshold voltages (Vth) of the completed thin film transistors are greatly varied. For example, data shown in FIGS. 4A and 4B are results examined with respect to a variation in threshold voltages of thin film transistors formed on an SOI substrate. In the drawings, a black circular mark indicates the case where a laminate structure of a silicon nitride film (SiN) and an acrylic film is used for the interlayer insulating film. In addition, an outline triangular mark in the drawings indicates the case where a laminate structure of a silicon nitride oxide film (SiNO) and a silicon oxynitride film (SiON) is used for the interlayer insulating film. In any case, the dry etching technique is used for the formation of the contact hole. Note that “SiNO” and “SiON” are separately used according to the meaning in which the former contains the amount of nitrogen larger than oxygen and the latter contains the amount of oxygen larger than nitrogen.
The data shown in FIGS. 4A and 4B are graphs obtained by evaluating a variation in threshold voltages using statistical processing. The ordinate indicates a channel length (carrier moving length) and the abscissa indicates a Vth variation. In recent years, “quartile deviation” has been known as statistical processing. The quartile deviation is a difference between a value of 25% and a value of 75% in a normal probability graph and has been noted as statistical processing which is not influenced by an abnormal value. The present applicant defines, based on the quartile deviation (which is also called 25 percentile deviation), a difference between a value of 16% and a value of 84% as 16 percentile deviation, and plots its value as “a Vth variation” in the abscissa. Note that the 16 percentile deviation corresponds to ±σ in a normal probability distribution. Thus, values, which are assumed as ±3σ by respectively multiplying by factors, are used for data plotting. When an acrylic film is used as an interlayer insulating film, as seen from the data, a variation in an n-channel TFT is about 4 times and a variation in a p-channel TFT is about 2 times those of the case not using the acrylic film. Thus, it is apparent that a variation is large in the case where the acrylic film is used. The present applicant estimates that a charge is captured in the acrylic film by plasma damage in dry etching, thereby providing a cause of varying a threshold voltage.